Freezing point lowering compositions are in widespread use for a variety of purposes, especially to reduce the freezing point of an aqueous system so that ice cannot form or formed ice can be melted.
Freezing point lowering compositions depend for their effectiveness upon the molar freezing point lowering effect, the number of ionic species which are made available and the degree to which the composition can be dispersed in the liquid phase in which the formation of ice is to be precluded and/or ice is to be melted.
Perhaps the simplest ice-melting substance is common salt (sodium chloride) which has widespread applicability for the melting of ice on road surfaces or the like, the salt dissolving in an available liquid in contact with ice and thereby forming a solution with a lower freezing point than the ice itself so that the ice is melted.
Typical solutions of low freezing points include brines and ethylene glycol solutions, the latter having been used for many years as automobile cooling system antifreeze.
Brines are used to transfer heat at temperatures below the normal freezing point of water and the ethylene glycol solutions can be employed in heat transfer systems for automobiles and the like in regions in which the temperature may fall below the normal freezing point of water.
Thus it will be apparent that freezing point lowering compositions have widespread applicability, ranging from ice melting applications already mentioned, to heat transfer applications, and to applications in which it is vital to maintain a liquid in the unfrozen state, e.g. as in a fire extinguisher.
Such antifreezing and ice dissolving compounds are used for lowering the freezing point of aqueous solutions for the purpose of avoiding solid ice formations on the one hand, and for dissolving ice already formed on the other. So far have become known and widely used for such media inorganic salts like sodium chloride, magnesium chloride, calcium chloride, potassium phosphates, sodium phosphates, ammonium phosphates, ammonium nitrate, alkaline earth nitrates, magnesium nitrate, ammonium sulphate, alkali sulphates and organic compounds such as low-molecular alcohols, glycols, glycerine, lactates and urea.
Besides the freezing point lowering effect on water resulting from the use of those products, a question of particular significance is the corrosive action of the media on various materials as well as that of environmental protection.
It is well known that so far the most widely used and least expensive ice dissolving and antifreezing compounds have been sodium chloride and/or calcium or magnesium chloride. However, the chlorine contents of aqueous solutions of these salts cause heavy corrosion to iron and other metallic materials, concrete and brickwork as well as serious damages to plants.
It is also a known fact that chlorine present in such de-icing salts or salt solutions hampers the water absorption in roots of trees and low-grade plants. Chloride penetrates them into leaves, causing its harmful effect there. Leaves turn brown, starting from their edges, and will fall of prematurely. Under rain and melting snow, chlorides are washed out again from the fallen leaves, and the harmful chloride circulation begins again. Moreover, damage may result to the paws of domestic animals and human footgear and clothing. Also other known inorganic salts may contribute to environmental dangers as well as increase the corrosion risk to materials.
When using known organic ice dissolving and antifreezing compounds, the danger of corrosion may be averted more early as there are many organic compounds of non-aggresive effect on materials and by inclusion of inhibitors, they may be adjusted for corrosion prevention particularly with respect to metals.
Apart from economic aspects--more organic compounds are substantially more expensive than equal freezing point lowering compounds of inorganic nature--the problem of pollution control must be dealt with.
For example, the monoethylene glycol used frequently in motor vehicles as an antifreeze is particularly detrimental to sewage treatment processes.
Alcohols, methyl alcohol, may also have toxic effects and high volatility particularly in low molecular weight range and may be the cause of offensive smell and fire danger.
Furthermore, these substances produce no lasting bar to ice formation.
Because such mono- and polyhydric alcohols oxidize in the presence of atmospheric oxygen, acids may form, giving rise in turn to increased corrosion of materials. Therefore buffer substances like potassium phosphates or borax as well as metal corrosion inhibitors like benzo-, toltyl triazoles, phosphonates, alcanolic amino-phosphates or molybdates are often added to those organic antifreezing compounds.
A typical example for such an ice dissolving and antifreezing compound is contained in Canadian Pat. No. 969 345. Such blends are also used for safety improvements on take-off and landing runways at airports and can include mixes composed of formamide, urea, water and chromate (Canadian Pat. No. 981 440).
But according to current knowledge neither formamide nor chromate should be used any longer for such purposes on account of their injurious and/or damaging effect to health and sewage water respectively.
According to German patent document No. 1 459 639 blends of formamide or formamide derivatives, water-soluble monohydric alcohols and glycolic ether are recommended for providing quick melting on surfaces covered with ice and snow.
In Czechoslovak patent 184 118 mixtures of ethanol with urea and inhibitors are described as liquid de-icing agents for airfields and streets. The problems encountered with organic products containing therein low-grade alcohols have been described already above.
Low-molecular monocarboxylic acids were already recommended as antifreezing compounds, i.e. the alkali salts of formic, acetic, propionic and butyric acids as per U.S. Pat. No. 2,233,185. Mixtures of urea with salt of low-saturated monocarboxylic acids, preferably of formic and acetic acids are described in Austrian Pat. No. 271 389. But these materials also have disadvantages.
Furthermore German patent document No. 2 933 318 describes the use of sodium/potassium chlorides in combinations with CaO, CaCO.sub.3, MgO and/or MgCO.sub.3. Here too, are contained harmful chlorides together with insoluble lime, dolomite.
Part of the patent literature deals with corrosion reduction of a de-icing salt (NaCl) by adding, for example, 0.1-10% lime nitrogen as per German Pat. No. 2 847 350 which, although able to reduce metal corrosion, does not avoid the harmful effects of NaCl on plants etc. The same applies to thawing salt (containing 80-99.9 weight % chlorides) that includes a corrosion inhibitor as per German patent document No. 2 161 522.
Due to the problems with chloride containing de-icing and antifreezing compounds as described above, efforts have been made since long to use other substances like ammonium sulphate, urea, sodium nitrate. Such blends, including various corrosion inhibitors are described in Austrain Pat. No. 191 383 as well as in U.S. Pat. No. 2,980,620.
As an antifreezing compound particularly suitable for fire fighting liquids, blends of urea, ammonium chloride, phosphate, sulphate are described in French Pat. No. 2 102 933. Also in U.S. Pat. No. 3,624,243 mixes of urea and ammonium nitrate with ethylic glycols are recommended as a de-icing liquid. However, all these compositions have disadvantages in use. It is known that urea as such becomes inoperative as a de-icing agent at tempertures of less than -8.degree. C. (cryohydric point -11.5.degree. C.).
Ammonium salts, however, that may produce lower ice dissolving temperatures attach concrete. Non-aggressive waters should have an NH.sub.4.sup.+ content of less than 15 mg per liter H.sub.2 O, a value which cannot be achieved in practice with the use of such salts as de-icing agents.
For non-aggressive waters the sulphate contents also should amount to less than 200 mg SO.sub.4.sup.= per liter water in case of Portland cement, to less than 400 mg in case of blast furnace slag cement, and to less than 600 mg SO.sub.4.sup.= for cement having an increased sulphate resistance (max. 3% tribasic calcium aluminate). When using water-soluble sulphates as de-icing agents these values can also not be met in practice. Ecologically, phosphates can be very damaging, and in many countries their amount contained in sewage is increasingly restricted.
The use of nitrates is not desirable both for reason of danger to sewage and dangerous state in dry condition, particularly in the presence of organic substances (explosion hazard).
From the foregoing it will be apparent that, while a large number of freezing-point-lowering systems have been developed heretofore and many of the problems attacked by the present invention have been recognized and treated with concern heretofore, a fully satisfactory freezing-point-lowering composition has not been developed because environmental hazards, explosion dangers, corrosion and the like have not fully been dealt with.